1. Field of the Invention
The present invention relates to a degradable polymer having resulting from the combination of a star-shaped polymer which is obtained by carrying out dehydration condensation of hydroxycarboxylic acid with polycarboxylic acid or anhydride thereof having three and more carboxyl groups or aliphatic polyhydric alcohol having three and more hydroxyl groups as a base compound, with aliphatic polyhydric alcohol having two and more hydroxyl groups or polycarboxylic acid or anhydride thereof having two and more carboxyl groups. The present invention also relates to a process for preparing the degradable polymer.
The degradable polymer of the invention has a high melt tension while maintaining an equivalent or higher transparency compared with aliphatic polyesters prepared by conventional techniques, for example, aliphatic polyester obtained by polycondensation of aliphatic polycarboxylic acid and polyhydric alcohol and aliphatic polyester obtained by polycondensation of aliphatic hydroxycarboxylic acid. The degradable polymer of the invention exhibits excellent processing ability such as in blow molding, foaming, inflation molding and other various processing methods.
Further, the degradable polymer of the invention is characterized by less discoloration compared to a copolymer prepared from polysaccharide as a polyfunctional base compound and aliphatic polyester, for example, aliphatic polyester obtained by polycondensation of aliphatic polycarboxylic acid with aliphatic polyhydric alcohol and aliphatic polyester obtained by polycondensation of hydroxycarboxylic acid.
The degradable polymer of the invention can be processed by utilizing the favorable properties into various molded article and work pieces, and is useful as a substitute of medical materials and general purpose resins.
2. Description of the Related Art
In recent years, waste disposal has become a problem in connection with environment protection. Specifically, molded articles and work pieces of general purpose polymer materials lead to problems in land fills, because they are lacking in ability to degrade or disintegrate by the action of microorganisms and semipermanently remain in soil as extraneous matter. They also cause elution of plasticizers and other additives and contaminate the environment. Further, in the case of incinerating these matters as refuse, the damage of the incinerator due to a great quantity of heat generated by combustion and numerous ill effects of discharged smoke and exhaust gas on the atmospheric pollution, ozone layer destruction, warming of the earth's atmosphere and acid rain have been noted.
In view of this situation, a polymer material having excellent degradability and toughness at the same time has been strongly desired. Nevertheless, the polymer material which can conform to such demand has not yet been supplied.
Conventionally, polyhydroxycarboxylic acid and polysaccharide have a characteristic of hydrolyzing with ease in the presence of water. When these polymers are used for a general purpose resin, they degrade without contaminating the environment after waste disposal, so they are friendly to the environment. When these polymers are left in a living body as a medical material, they are degraded and absorbed in the living body without giving toxicity to the living body after attaining the object, so they are thus friendly to the living body. These excellent properties have already received attention.
For example, polylactic acid and cellulose acetate have a characteristic of hydrolyzing with ease in the presence of water. In the case of using these polymers as a general purpose resin, these polymers are easy on the environment because of degradation without contamination of the environment after waste disposal. In the case of leaving the polymers in a living body as a medical material, these polymers do not give an adverse effect on the living body, can be decomposed and absorbed in the living body without poisoning after attaining the object, and thus have a characteristic of being easy on the living body.
However, in the case of processing into molded articles or work pieces such as a film and filament, polylactic acid is excellent in transparency and simultaneously has problems of brittleness, high hardness, lack of flexibility and low melt tension. Cellulose acetate can be used for a cigarette filter and photographic base film, where it has suffered the disadvantage of requiring an extraordinarily great amount of plasticizer.
In view of the foregoing technical background, techniques have been developed in order to overcome these problems. One technique is to radially add high molecular weight degradable polymers as side chains to a polyfunctional base compound by ring-opening polymerization of lactide, that is, a cyclic dimer of lactic acid and cyclic monomers such as lactones, for example, .epsilon.-caprolactone.
For example, BP 2,145,422 has disclosed a technique on a polymer obtained by adding polylactic acid or polyhydroxycarboxylic acid as a side chain to hydroxyl group of polyhydric alcohol such as sugar or sugar alcohol. More specifically, the disclosed technique relates to an ester of polyhydric alcohol which involves ester bonds formed by hydroxyl groups of polyhydric alcohol and carboxyl groups of polylactic acid or copolylactic acid. That is, the disclosed technique relates to the polymer which is obtained by esterifying polyhydric alcohol such as glucose or a derivative thereof having a molecular weight of 20,000 or less with polylactic acid or a derivative thereof or with copolylactic acid or a derivative thereof having a molecular weight of 5,000 or more.
The ester of polyhydric alcohol has a relatively low molecular weight and is suitable for being applied to a slow release medicine and other DDS.
U.S. Pat. No. 5,210,108 has disclosed a technique relating to a rigid resin foam of a star-shaped polymer which is obtained by using a polyfunctional base compound having 3-100 amino or hydroxyl groups and 5-10,000 carbon atoms and by radially adding two or more high molecular weight arms (side chains) to the polyfunctional base compound.
The high molecular weight degradable side chain is a segment compound of polyhydroxycarboxylic acid such as polylactic acid, polyglycolide or polycaprolactone, and has a specific structure that the portion close to the polyfunctional base compound is composed of an amorphous segment and the portion remote from the polyfunctional base compound is composed of a semi crystalline segment.
Further, Japanese Laid-Open Patent HEI 6-287279 has disclosed a preparation process of a lactide-based graft copolymer obtained by reacting cellulose ester or cellulose ether with lactide.
That is, the technique is characterized by carrying out ring opening graft copolymerization of lactide with cellulose ester or cellulose ether in the presence of an esterification catalyst and can provide a preparation process of a lactide-based graft copolymer which is excellent in transparency, degradability, thermoplastic property and laminating ability. The above mentioned prior techniques are ring opening polymerization methods and these ring-opening polymerization methods require conventionally many process steps compared with direct dehydration condensation and the cyclic dimer is also expensive. Further, the technique has suffered problems of tendency to coloration in the step of adding the degradable high polymer side chain to polysaccharide, the polyfunctional base compound.
In such a technique for providing the polyfunctional base compound with a degradable high molecular weight side chain by ring opening polymerization of a cyclic monomer such as lactide and lactones or direct dehydration condensation of a hydroxycarboxylic acid such as lactic acid in place of lactide or lactones, when the amount of the polyfunctional base compound is increased, the equivalent ratio of the hydroxyl group or amino group to the carbonyl group or carboxyl group deviates from 1 in the reaction and thus it is generally difficult to obtain a degradable polymer having a high molecular weight sufficient to exhibit high melt tension. The trend is increased in particular, when the polyfunctional base compound is low molecular compound.
The present inventors have already disclosed in U.S. Pat. No. 5,310,865 a technique for preparing a high molecular weight aliphatic polyhydroxycarboxylic acid in high purity and a technique for preparing a film, filament and molded article which comprise said polyhydroxycarboxylic acid and have excellent strength by directly carrying out dehydration condensation of aliphatic hydroxycarboxylic acid without using a cyclic monomer.
The adoption of the technique disclosed in U.S. Pat. No. 5,310,865 has for the first time enabled one to obtain a high molecular weight aliphatic polyhydroxycarboxylic acid by directly carrying out dehydration condensation of the intact aliphatic hydroxycarboxylic acid such as lactic acid and other non-cyclic monomers and not by way of the cyclic dimers of the same.
One object of the invention is to obtain a degradable polymer having properties described below by further developing the technical concept disclosed in U.S. Pat. No. 5,310,865.
1) Much less coloration as compared with copolymers obtained by using polysaccharide as a polyfunctional center compound. PA1 2) High melt tension as compared with common aliphatic polyester. PA1 3) Excellent in foaming property and other processing properties as compared with common aliphatic polyester. PA1 4) Equivalent or higher transparency as compared with common aliphatic polyester. PA1 (b-1) polycarboxylic acid having three or more carboxyl groups, PA1 (b-2) anhydride of polycarboxylic acid having three or more carboxyl groups, and PA1 (b-3) aliphatic polyhydric alcohol having three or more hydroxyl groups, and a component (C): one or more compound which can conduct an esterification reaction with the component (B) and is selected from the group consisting of: PA1 (c-1) polycarboxylic acid having two or more carboxyl groups, PA1 (c-2) anhydride of polycarboxylic acid having two or more carboxyl groups, and PA1 (c-3) aliphatic polyhydric alcohol having two or more hydroxyl groups. PA1 1) After dehydration condensation of component (A) and component (B) in the presence of a catalyst, component (C) is added and the dehydration condensation is further continued to obtain a degradable polymer having a high weight average molecular weight. PA1 2) After dehydration condensation of component (A) and component (C) in the presence of a catalyst, component (B) is added and the dehydration condensation is further continued to obtain a degradable polymer having a high weight average molecular weight. PA1 3) After dehydration condensation of component (A) in the presence of a catalyst, component (B) and component (C) are added and the dehydration condensation is further continued to obtain a degradable polymer having a high weight average molecular weight. In this case, component (B) and component (C) can be added at the same time or separately with a time internal. PA1 4) Component (A), component (B) and component (C) are simultaneously charged and dehydration condensation is carried out in the presence of a catalyst to obtain a degradable polymer having a high weight average molecular weight. PA1 1) a method for previously charging the organic solvent in excess and removing a portion thereof from the reaction system simply, PA1 2) a method for drying the organic solvent by using other organic solvents, PA1 3) a method for removing a portion of the solvent from the reaction system, dehydrating outside the reaction system by treatment with a drying agent or distillation in order to obtain the organic solvent having the amount of water less than that of the removed organic solvent and returning again to the reaction system.